Merge branch 'pb/complete-and-document-auto-merge-and-friends'

Document more pseudo-refs and teach the command line completion machinery to complete AUTO_MERGE. * pb/complete-and-document-auto-merge-and-friends: completion: complete AUTO_MERGE Documentation: document AUTO_MERGE git-merge.txt: modernize word choice in "True merge" section completion: complete REVERT_HEAD and BISECT_HEAD revisions.txt: document more special refs revisions.txt: use description list for special refs